Swift object pool design sample

A generic object pool in Swift

The object pool sample is a creational design sample. The principle concept behind it’s that first you create a set of objects (a pool), then you definitely purchase & launch objects from the pool, as a substitute of continually creating and releasing them. 👍

Why? Efficiency enhancements. For instance the Dispatch framework makes use of an object pool sample to present pre-created queues for the builders, as a result of making a queue (with an related thread) is an comparatively costly operation.

One other use case of the object pool sample is employees. For instance you must obtain lots of of pictures from the online, however you’d prefer to obtain solely 5 concurrently you are able to do it with a pool of 5 employee objects. In all probability it is going to be lots cheaper to allocate a small variety of employees (that’ll truly do the obtain job), than create a brand new one for each single picture obtain request. 🖼

What concerning the downsides of this sample? There are some. For instance if in case you have employees in your pool, they could include states or delicate consumer knowledge. You must be very cautious with them aka. reset every thing. Additionally in case you are working in a multi-threaded atmosphere you must make your pool thread-safe.

Right here is a straightforward generic thread-safe object pool class:

import Basis

class Pool<T> {

    non-public let lockQueue = DispatchQueue(label: "pool.lock.queue")
    non-public let semaphore: DispatchSemaphore
    non-public var objects = [T]()

    init(_ objects: [T]) {
        self.semaphore = DispatchSemaphore(worth: objects.rely)
        self.objects.reserveCapacity(objects.rely)
        self.objects.append(contentsOf: objects)
    }

    func purchase() -> T? {
        if self.semaphore.wait(timeout: .distantFuture) == .success, !self.objects.isEmpty {
            return self.lockQueue.sync {
                return self.objects.take away(at: 0)
            }
        }
        return nil
    }

    func launch(_ merchandise: T) {
        self.lockQueue.sync {
            self.objects.append(merchandise)
            self.semaphore.sign()
        }
    }
}


let pool = Pool<String>(["a", "b", "c"])

let a = pool.purchase()
print("(a ?? "n/a") acquired")
let b = pool.purchase()
print("(b ?? "n/a") acquired")
let c = pool.purchase()
print("(c ?? "n/a") acquired")

DispatchQueue.world(qos: .default).asyncAfter(deadline: .now() + .seconds(2)) {
    if let merchandise = b {
        pool.launch(merchandise)
    }
}

print("No extra useful resource within the pool, blocking thread till...")
let x = pool.purchase()
print("(x ?? "n/a") acquired once more")

As you’ll be able to see the implementation is just some traces. You’ve the thread protected array of the generic pool objects, a dispatch semaphore that’ll block if there aren’t any objects obtainable within the pool, and two strategies with a view to truly use the thing pool.

Within the pattern you’ll be able to see that if there aren’t any extra objects left within the pool, the present queue will likely be blocked till a useful resource is being freed & prepared to make use of. So be careful & do not block the primary thread by chance! 😉